1. Field of the Invention
The present invention relates to mechanical fittings and tolerancing, in particular to fittings found in shearable devices, and more particularly to pyrotechnically activated devices such as valves.
2. Background of the Invention
Pyrotechnic valves are commonly used in rockets for valves which must be opened reliably one time. Typically, these valves contain a seal tube which holds a pressurized gas, and this seal tube is pyrotechnically sheared to open the sealed end of the seal and start the flow of gas. The seal tube typically needs to be very strong in order to support the high gas pressure, such as by helium gas, dictated by the design. Also, initiator redundancy is typically needed to promote reliability. Moreover, for reliability, for example, these devices are typically built with two pyrotechnic initiators for redundancy. Also, the valve typically must be able to function using only one initiator loaded to 75% or with two initiators loaded to 125%. Such valve requirements create a range of initial ballistic gas pressures with a ratio of approximately 4:1, for example. Further, peak pressures are limited by the ability of O-rings and other gas-tight fittings to contain such pressures.
One potential method of addressing the above described needs in a pyrotechnic valve is to have the piston supply a much larger force at the beginning of a stroke. This could be accomplished, for example, by either increasing the initial ballistic gas pressure or by increasing the area of the piston that is acted on by the gas. However, such pressure increase in this first potential method can be unacceptable because of a need for initiation redundancy in the device.
A second potential method of addressing the above desired needs in a pyrotechnic valve is to increase the piston area, which can be undesirable for two reasons. First, increasing the piston area can make the piston physically large and heavy. Second, more propellant charge or possibly even an additional booster charge would be required in the initiators. These characteristics can be undesirable from a design standpoint and can generally raise the cost of the pyrotechnic valve device.
Moreover, the construction of pyrotechnic valve devices must be done to exacting tolerances. Pyrotechnic valve devices, such as those used in rocketry, for example, are often subject to considerable vibration. If there is “play” between the parts, vibrational damage can result.